John Calvin Commentary Isaiah 7:15

"Butter and honey shall he eat, when he knoweth to refuse the evil, and choose the good." — Isaiah 7:15 (ASV)

Butter and honey shall he eat. Here the Prophet proves the true human nature of Christ, for it was altogether incredible that He who was God should be born of a virgin. Such a prodigy was repugnant to ordinary human judgment.

To prevent us from thinking that the Prophet's imagination now presents some apparition to us, he describes the marks of human nature to show, through them, that Christ will actually appear in flesh, or in the nature of man. This means that He will be reared in the same manner that children commonly are.

The Jews had a different way of rearing children from what we follow, for they used honey, which is not so customary among us. To this day, they still retain the custom of having a child taste butter and honey as soon as it is born, before suckling.

That he may know. This means until He arrives at that age when He can distinguish between good and evil, or, as we commonly say, until the years of discretion.

The Hebrew letter ל (lamed) denotes the term and period up to which He will be reared in the manner of a child, and this contributes even more to prove the reality of His nature. He therefore means understanding and judgment, such as is obtained when the period of childhood is past.

Thus we see how far the Son of God condescended for our sake, so that He not only was willing to be fed with our food but also, for a time, to be deprived of understanding and to endure all our weaknesses (Hebrews 2:14). This relates to His human nature, for it cannot apply to His divinity. Luke testifies about this state of ignorance, in which Christ was for a time, when he says:

And he grew in wisdom, and in stature,
and in favor with God and with man. (Luke 2:52)

If Luke had merely said that Christ grew, it might have been supposed to mean with men; but he expressly adds, with God. Christ must therefore have been, for a time, like little children, so that, as far as relates to His human nature, He was deficient in understanding.